News

Ageing stars are slow on the outside but fast on the inside

By Katynna Gill

8 December 2011

What will our Sun look like in five billion years? It will have a fast spinning core and slow spinning surface, according to an international team of scientists, including University of Sydney astronomers Professor Tim Bedding and Dr Dennis Stello. The team has discovered that old stars called 'red giants' have slowed down on the outside, while their cores spin at least ten times faster than their outer layers.

The finding, published in the prestigious journal Nature, gives an insight into what our Sun will look like in five billion years, when it develops into a red giant.

Professor Tim Bedding and Dr Dennis Stello, from the School of Physics, with international colleagues, have found that old stars called 'red giants' have slowed down on the outside, while their cores spin at least ten times faster than their outer layers. Image credit: Paul G. Beck, Leuven University, Belgium.

"The heart of a star determines how it evolves and understanding how a star rotates deep inside helps us to understand how stars like our Sun will grow old," said Professor Tim Bedding, from the University of Sydney's School of Physics.

Using NASA's Kepler space telescope, the team observed deep inside ageing red giants to make their discovery of the difference in rotation rate between the core and outer layers of the stars.

The team, led by Paul Beck from Leuven University in Belgium, analysed waves inside the stars, which appear as rhythmic variations in the surface brightness of the stars. The effect of rotation on the frequencies of the waves is so small that it took the team nearly two years of almost continuous data gathering from the Kepler satellite to make their discovery.

"Red giants were once stars like our Sun, but as they age, their outer layers expand to more than five times their original size and cool down significantly, so they look red," explained Dr Dennis Stello, from the University of Sydney's School of Physics.

"The opposite actually happens to the cores of red giants, as the core contracts and becomes extremely hot and dense," said Dr Stello.

"We've just discovered that the core spins much faster than the outer layers in these old stars, which makes sense when you consider what happens to other spinning things like, say, an ice skater performing pirouettes.

"A spinning ice skater will slow down if their arms are stretched far out, like the expanded outer layers of the red giants. The ice skater will spin faster if their arms are pulled tightly to the body, like the fast spinning contracted core of red giants."

The Kepler space telescope is one of NASA's most successful space missions and is searching in the constellation Cygnus for potentially habitable planets by focussing on those similar in size to Earth.

Our Sun (left) will develop into a red giant (right) as it ages over the next five billion years. Red giant stars have outer layers expanded to more than five times their original size and cool down significantly, so they look red. Image credit: Paul G. Beck, Leuven University, Belgium.

"Kepler is able to detect variations in a star's brightness of only a few parts in a million, so its measurements are ideally suited todetect the tiny brightness fluctuations of stars," explained Dr Stello.

"We study these variations in brightness to work out what's going on deep inside stars. It's called asteroseismology - just as geologists use earthquakes to explore Earth's interior, we use star quakes to explore the interiors of stars," said Dr Stello.

Different waves probe different parts of the star and by a detailed comparison of the depth to which these waves travel inside the star, the team found a dramatic increase of the rotation rate towards the stellar core.

In addition to helping us understand how stars age, asteroseismology will help Kepler's mission of discovering Earth-sized planets outside our solar system by characterising the host stars around which these planets orbit.